1. Field of the Invention
The present invention relates to a light modulation apparatus and a light modulator control method of an external modulation system used for light communication systems, and more particularly to a light modulation apparatus and a light modulator control method using a Mach-Zehnder type light modulator.
2. Description of Related Art
In recent years, as the light communication systems have advanced, a light modulation apparatus capable of stably outputting a high frequency output light signal (light pulses) has been demanded. For example, a light modulation apparatus equipped with an external modulator such as an electro-absorption (EA) modulator, an LN modulator using a material having an electro-optical effect (Pockels effect) of lithium niobate (LiNbO3) or the like as a substrate, or the like has been put to practical use.
FIG. 3 is a diagram showing the configuration of a light modulation apparatus 100 using the EA modulator mentioned above.
As shown in FIG. 3, the light modulation apparatus 100 is composed of an EA modulator 21 modulating and outputting light having entered therein from a light source 11 such as a laser diode or the like, and a driver 22 for EA modulator, which controls the operation relative to the light modulation of EA modulator 21 according to the logical level of an electronic signal input from the outside.
The EA modulator 21 is an electro-absorption type light modulator performing absorption or transmission of light according to a control signal input from the driver 22 for EA modulator. When an input electric signal input from the outside has a low level, the driver 22 for EA modulator outputs a control signal to make the EA modulator 21 absorb the light input from the light source 11 so as to make the output light output from the EA modulator 21 be in an extinction (off) state. Moreover, when an electric signal input from the outside has a high level, the driver 22 for EA modulator outputs a control signal to make the EA modulator 21 transmit the light input from the light source 11 so as to make the output light output from the EA modulator 21 be in a lighted (on) state.
Although the EA modulator 21 has an advantage of not generating any temperature drifts and any drifts with time, the EA modulator 21 has a problem of the inferiority of the extinction ratio which is a ratio of the light intensity of an on-state to the light intensity of an off-state of an output light signal, and has a problem of the inferiority of a waveform quality of the output light signal. Consequently, there is the possibility of generating an error at the time of a light communication.
FIG. 4 is a diagram showing the configuration of a light modulation apparatus 100 using the LN modulator mentioned above.
As shown in FIG. 4, the light modulation apparatus 100 is composed of an LN modulator 12 modulating the light input from the light source 11 such as a laser diode or the like to output the modulated light as an output light signal, a driver 13 for LN modulator, which controls the operations relative to the light modulation of the LN modulator 12 according to the logical level of the input electric signal input from the outside, an optical branching circuit 15 branching the output light signal output from the LN modulator 12, and a bias control circuit 19 adjusting a bias point of the LN modulator 12 based on the output light signal branched by the optical branching circuit 15.
The LN modulator 12 is a Mach-Zehnder type light modulator, which branches the light entered from the light source 11 into two lights, changes the branched lights in correspondence with the control signal input from the driver 13 for LN modulator, and multiplexing the two lights into one light again after that to output the multiplexed light. When an input electric signal input from the outside is the low level, the driver 13 for LN modulator applies a control signal to change the phases of the two lights branched in the LN modulator 12 to be different from each other by the degree of π, to the LN modulator 12 as an RF voltage, and thereby makes the two lights be multiplexed in the opposite phase to each other to make the output light signal be in an extinction (off) state. Moreover, when an electric signal input from the outside is the high level, the driver 13 for LN modulator applies a control signal to change the phases of the two lights branched in the LN modulator 12 to agree with each other, to the LN modulator 12 as the RF voltage, and thereby multiplexes the two lights in the same phase to make the output light signal be in the lighted (on) state.
Hereupon, as the control signal input from the driver 13 for LN modulator, a no return to zero (NRZ) signal and a return to zero (RZ) signal, which are severally continuously generated and have an almost constant mark rate (a rate of the low level to the high level), are used. Moreover, the light modulation apparatus can be used in a mode in which the capacitance of a capacitor or the like are connected between the LN modulator 12 and the driver 13 for LN modulator because the connection brings about the advantages of arbitrary setting the bias value in the inside of the driver 13 for LN modulator, and the like.
Although the light modulating apparatus using the LN modulator has the advantages of being excellent in the extinction ratio and of the realizability of a wide area ideal light modulation having no chirping in comparison with those of the light modulating apparatus using the EA modulator, the light modulating apparatus using the LN modulator has a problem of the deterioration of the accuracy of the output light signal thereof because temperature drifts and drifts with time are generated. Accordingly, the bias control circuit 19 suppresses the generation of the drifts by applying a bias voltage (DC voltage) so that the middle point (middle point between the maximum output and the minimum output) of a characteristic curve of light modulation by the LN modulator 12 may become the bias point based on the output light signals which have been branched by the optical branching circuit 15. Moreover, as conventional techniques about the adjustment of the bias point of a light modulator, Japanese Patents No. 2642499 and No. 3398929 have been proposed, for example.
There is a demand of operating the light modulation apparatus mentioned above in a burst mode on account of a system and of dealing with further speeding up. In the burst mode, states can be divided broadly into the states (hereinafter each referred to a burst state) in each of which a series of output light signals in the on-state (hereinafter each referred to as a burst signal) is intermittently output, and the states (hereinafter each referred to as a burst pause state) in each of which an output light signal in the off-state continues (see FIG. 2). In this case, because the LN modulator cannot be efficiently operated when the center point of the characteristic curve of the light modulation mentioned above is set as the bias point, it is necessary to set the bias point at a point where the light intensity of the output light signal becomes the weakest extinction state (glimmer).
However, in the techniques described in the Japanese Patents No. 2642499 and No. 3398929 mentioned above, because the operation at the time of the burst mode, in which the mark rate is not constant, is not taken into consideration, there is a problem in which it is impossible to continue controlling the point where the light intensity of the output light signal becomes the weakest extinction state as the bias point, and to perform the efficient modulation of the output light signal.